Animation nodes to shader nodes
As stated in https://blender.stackexchange.com/a/168589/86891, you have to access a specific object's property in order to change each individual object if they share the same material. Each object has a few specific properties : Render pass index, viewport color, etc.

But you don't have to use the Animation Nodes for this since the shader nodes contains a "Random" -per object info which will act exactly as what you want to achieve.



However if you really want to use AN, use this setup in your shader :



And in your AN tree :